The present invention relates to a control circuit for protecting an excess discharge of a battery and, particularly to, a battery protection circuit housed in a so-called battery pack, in a battery charger, or in devices driven by a battery.
Self-discharge amounts of battery packs are very small even after they have been left for a long time. Under normal room temperature circumstances, the battery packs are never entered into the excess discharge region even after they had been left about a year. If however the battery packs are not in use and left under the condition that they are electrically connected to some audio and video devices, such as a video camera, then a dark current of about 2 to 20 mA flows even though the power switch of the video camera has been turned off. As a consequence, the battery packs reach the excess discharge region within about one to six months. The battery packs, in worst cases, will not recover from the excess discharge state even with help of the ordinary battery charger.
To solve the above-mentioned problem, a proposed battery pack houses a battery protection circuit to protect the battery from overcharge and excess discharge. Specifically, the battery protection circuit includes an electronic switch connected in series between one of the positive and negative connection terminals connected with a load or a battery charger and a battery cell. The battery protection circuit can protect the battery pack from overcharge or excess discharge by use of this electronic switch.
In the above-mentioned battery protection circuit, when the battery pack is discharged by the load connected to the positive and negative connection terminals or the battery pack is charged by the battery charger connected to the positive and negative connection terminals, a current flows in the battery protection circuit in the respective opposite directions. Accordingly, the above-mentioned electronic switch uses a field effect transistor (FET) which enables a current to flow in the two directions opposite to each other. Alternatively, the electronic switch uses transistors of positive and negative directions connected in parallel to each other to enable a current to flow in the two directions opposite to each other when the battery pack is discharged or charged.
Further, in order to protect the battery pack from overcharge, the battery protection circuit uses some suitable means, such as a thermostatic switch for detecting a temperature of a battery cell and opening and closing the circuit based on a detected result. Further, a detector for detecting charge and discharge includes a resistor connected in series to a power supply. A resistance value of this resistor is selected to be sufficiently small so as not to affect the output of the detector. To avoid the battery from being discharged excessively, the charge and discharge detector is formed of a suitable means, such as a CMOS (complementary metal oxide semiconductor) circuit whose power consumption is small.
However, the above-mentioned battery protection circuit uses the FET, the thermostatic switch, the resistor having low resistance value and the CMOS circuit all of which are relatively expensive circuit elements. Moreover, in order to drive the transistors connected in parallel under predetermined conditions, respectively, there has to be provided a complicated circuit arrangement. Therefore, if the battery pack incorporates therein the above battery protection circuit, then it is unavoidable that the battery pack becomes expensive.
Further, when the battery protection circuit uses the temperature detecting circuit, such as the thermostatic switch in order to prevent overcharge, if the battery protection circuit is placed in the abnormal load state, such as short-circuits repeatedly occurring between the positive and negative connection terminals, then the temperature detecting circuit chatters and an operation temperature exceeds a predetermined temperature. There is then the risk that the battery protection circuit will not be operated normally.